2.1. Diagnostic Assays
There is a strong economic need for rapid sensitive diagnostic technologies. Diagnostic technologies are important in a wide variety of economic markets including health care, research, agricultural, veterinary, and industrial marketplaces. An improvement in sensitivity, time required, ease of use, robustness, or cost can open entirely new diagnostic markets where previously no technology could meet the market need. Certain diagnostic technologies may possess high sensitivity but are too expensive to meet market needs. Other techniques may be cost effective but not robust enough for various markets. A novel diagnostic technique which is capable of combining these qualities is a significant advance and opportunity in the diagnostics business.
There are a number of different analytical techniques used in diagnostic applications. These techniques include radioactive labeling, enzyme linked immunoassays, chemical calorimetric assays, fluorescence labeling, chemiluminescent labeling, and electrochemiluminescent labeling. Each of these techniques has a unique combination of sensitivity levels, ease of use, robustness, speed and cost which define and limit their utility in different diagnostic markets. These differences are in part due to the physical constraints inherent to each technique. Radioactive labeling, for example, is inherently non-robust because the label itself decays and the disposal of the resulting radioactive waste results in economic, safety and environmental costs for many applications.
Many of the sensitive diagnostic techniques in use today are market-limited primarily because of the need for skilled technicians to perform the tests. Electrochemiluminescent procedures in use today, for example, require not only skilled technicians but repeated washing and preparatory steps. This increases both the costs and the need for waste disposal. Novel diagnostics which simplify the testing procedures as well as decrease the cost per test will be of great importance and utility in opening new markets as well as improving performance in existing markets.
2.2. Electrochemiluminescence
Electrochemiluminescence (“ECL”) is the phenomena whereby an electrically excited species emits a photon (see, e.g., Leland and Powell, 1990 J. Electrochem. Soc. 137(10):3127-3131). Species from which ECL can be induced are termed ECL labels and are also referred to herein as TAGs. Commonly used ECL labels include: organometallic compounds where the metal is from, for example, the noble metals of group VIII, including Ru-containing and Os-containing organometallic compounds such as the Ru(2,2′-bipyridine)32+ moiety (also referred to as “Rubpy” or TAG1), disclosed, e.g., by Bard et al. (U.S. Pat. No. 5,238,808). “TAG1” and “Rubpy” also refer to derivatives of Ru(2,2′-bipyridine)32+. Fundamental to ECL-based detection systems is the need for an electrical potential to excite the ECL label to emit a photon. An electrical potential waveform is applied across an electrode surface, typically a metal surface, and a counterelectrode (see e.g., U.S. Pat. Nos. 5,068,088, 5,093,268, 5,061,445, 5,238,808, 5,147,806, 5,247,243, 5,296,191, 5,310,687, 5,221,605). The ECL is promoted to an excited state as a result of a series of chemical reactions triggered by the electrical energy received from the working electrode. A molecule which promotes ECL of the TAG is advantageously provided, such as oxalate or, more preferably, tripropylamine (see U.S. Pat. No. 5,310,687).
The excitation of a TAG in an ECL reaction typically involves diffusion of the TAG molecule to the surface of an electrode. Other mechanisms for the excitation of a TAG molecule by an electrode include the use of electrochemical mediators in solution (Haapakka, 1982, Anal Chim. Acta, 141:263) and the capture of beads presenting TAG molecules on an electrode (PCT published applications WO 90/05301 and WO 92/14139). Alternatively, ECL has been observed from TAG that was adsorbed directly on the surface of working electrodes (U.S. Pat. No. 5,324,457), e.g., by non-specific adsorption (Xu et al., 1994, Langmuir, 10:2409-2414), by incorporation into L-B films (Zhang et al., 1988, J. Phys. Chem., 92:5566), by incorporation into self-assembled monolayers (Obeng et al., 1991, Langmuir, 7:195), and by incorporation into thick (micrometer) films (Rubenstein et al., 1981, J. Am. Chem. Soc., 102:6641). Similarly, Xu et al. (PCT published application WO 96/06946) have observed ECL from TAG molecules intercalated into DNA strands when such strands were adsorbed onto gold electrodes by interaction with aluminum centers immobilized on a self-assembled monolayer of alkanethiolates.
Various apparatus well known to the art are available for conducting and detecting ECL reactions. For example, Zhang et al. (U.S. Pat. No. 5,324,457) discloses exemplary electrodes for use in electrochemical cells for conducting ECL. Leventis et al. (U.S. Pat. No. 5,093,268) discloses electrochemical cells for use in conducting ECL reactions. Kamin et al. (U.S. Pat. No. 5,147,806) discloses apparatus for conducting and detecting ECL reactions, including voltage control devices. Zoski et al. (U.S. Pat. No. 5,061,445) discloses apparatus for conducting and detecting ECL reactions, including electrical potential waveform diagrams for eliciting ECL reactions, digital to analog converters, control apparatus, detection apparatus and methods for detecting current generated by an ECL reaction at the working electrode to provide feedback information to the electronic control apparatus.
2.3. Commercial ECL Assays
The light generated by ECL labels can be used as a reporter signal in diagnostic procedures (Bard et al., U.S. Pat. No. 5,221,605). For instance, an ECL label can be covalently coupled to a binding agent such as an antibody or nucleic acid probe. The ECL label/binding agent complex can be used to assay for a variety of substances (Bard et al., U.S. Pat. No. 5,238,808). The use of ECL in assays is reviewed in detail by, for example, Knight et al., 1994, Analyst, 119:879-890. In brief, the ECL technique may be used as a method of detecting in a volume of a sample an analyte of interest present in the sample in relatively small concentrations.
To date, all commercial ECL assays are carried out on centimeter scale electrode surfaces. The centimeter scale electrodes strike a balance between the enhanced magnitude of an ECL signal resulting from larger electrodes and the desirability of decreasing the total sample volume necessary for each assay. However, even centimeter scale electrodes fail to achieve the sensitivity required for many assays. In an attempt to overcome this problem, all commercial ECL systems further enhance sensitivity by using coated magnetic beads to capture ECL analytes or reagents. The beads are then moved adjacent to a working electrode for enhanced sensitivity.
However, the currently available technology has many limitations (primarily cost and complexity) that restrict its use in low cost assays employing disposable cartridges as well as its use in high throughput systems that perform multiple assays concurrently.
Leventis et al. (U.S. Pat. No. 5,093,268) has proposed a method of assaying more than one different analyte simultaneously by the use of different ECL labels for each analyte, each emitting photons at different wavelengths for each different analyte in a single assay. However, this technique is limited, for example, by the unavailability of a sufficient number of effective ECL labels radiating at different wavelengths and the need to optimize the chemical conditions for each ECL label. These practical constraints have prevented the commercialization of such multi-wavelength, multi-analyte ECL detection systems.
Commercial methods for conducting ECL assays also require that the assay cell, including the electrodes, must be cleaned by any one of a number of methods, including the use of dilute acids, dilute bases, detergent solutions, and so forth as disclosed, for example, by U.S. Pat. No. 5,147,806.
2.4. Objects of the Invention
It is therefore an object of the present invention to provide a novel, cost effective electrode and disposable for conducting ECL assays.
It is a further object of the present invention to provide a novel and cost effective system for conducting a plurality of ECL assays, either sequentially or simultaneously and in a preferred embodiment, providing built-in control standards for improved accuracy.
It is a further object of the present invention to provide a cassette comprising one or more supports suitable for conducting a plurality of simultaneous or sequential ECL assays that is also disposable.
It is a further and related object of this invention to reduce the time and cost of conducting individual assays for analytes of interest in biological samples.
It is still a further and related object of this invention to provide methods and apparatus for conducting a plurality of simultaneous assays for a plurality of analytes of interest in a single biological sample.